U.S. Pat. No. 2,628,160 (Stookey) discloses photonucleable glasses that are capable of being chemically sculptured. That is, the glasses disclosed in that patent are susceptive to the development of opacification in selected zones thereof due to the generation of crystals therein via the selective radiation of those zones by shortwave (conveniently ultraviolet) radiation followed by heat treatment. The patent notes that the great difference in solubility existing between the crystal phase and the precursor, i.e., uncrystallized, glass results in the ready leaching of the crystalline regions from the zones of glass.
U.S. Pat. No. 4,572,611 (Bellman et al.) discloses an optical device founded upon the glass compositions described in U.S. Pat. No. 2,628,160. Thus, Bellman et al. observed a particular phenomenon which occurs in the selectively opacifiable glasses described in that patent. They noted that differential densification of the glass occurs during the photonucleating and crystal generating process. Hence, the crystal-containing areas contract and thereby become denser when compared to the transparent, crystal-free (glass) areas which do not undergo change. That contraction results in the formation of a physically raised pattern having an unbroken surface. Bellman et al. found that the configuration of that raised portion, termed by them a "relief image," could be controlled by adjusting the initial radiation exposure of the glass, and so provided the opportunity for producing an optical pattern in the glass.
Hence, as explained in that patent, Bellman et al. employed an otherwise transparent mask having a pattern of opaque dots corresponding to a desired lens array. That is, the mask is transparent to the activating actinic radiation (commonly ultraviolet radiation), except for the pattern of dots which are opaque to the actinic radiation. Accordingly, where the glass is exposed through the mask, the exposed zones become nucleated, whereas the cylindrical areas under the dots are not exposed and, hence, are not nucleated. When the glass is subsequently heat treated to cause the growth of crystals in the exposed zones, the resultant crystal-containing zones contract and draw away from the unexposed, uncrystallized cylindrical areas. Surface tension causes the transparent areas to become raised protrusions which assume spherical shapes such that the glass article appears to have raised spherical surfaces with interstitial valleys.
Bellman et al. observed that any of the photosensitively opacifiable glasses described in U.S. Pat. No. 2,628,160 were operable in their invention and incorporated that patent in their disclosure for that teaching. That patent disclosed glasses wherein the crystal phases developed therein were selected from the group of lithium disilicate, lithium metasilicate, barium disilicate, and alkali metal fluoride. Nevertheless, because a lithium silicate photosensitively opacifiable glass was commercially available from Corning Glass Works (now Corning Incorporated) under Code 8603, Bellman et al. used it in their work.
U.S. Pat. No. 4,518,222 (Borrelli et al.) describes a modification/improvement upon the disclosure of U.S. Pat. No. 4,572,611. Thus, Borrelli et al. discovered that the characteristics displayed by the optical devices prepared in accordance with the method of Bellman et al. utilizing lithium-containing glasses could be altered by subjecting the devices to an ion exchange reaction wherein larger alkali metal ions, conveniently Na.sup.+ and/or K.sup.+ ions, from an external source are exchanged with Li.sup.+ ions in the glass. This ion exchange process produces both axial and radial concentrations of alkali metal ions in the lenses. That is, the level of Li.sup.+ ions within a surface layer of the lenses is less than that present in the interior of the lenses, and the level of exchangeable larger alkali metal ions is greater within the surface layer than that present in the interior of the lenses, with the distribution of the alkali metal ions being defined by both axial and radial concentration gradients. Such gradients, in turn, generate refractive index and dispersion gradients. Through careful control of those gradients, it is possible to correct aberrations in lens systems.
Borrelli et al. also discovered that, if the ion exchanged zone was heated to a temperature above the strain point of the glass, the optical power of the lenses could be significantly increased. The physical effect of the heat treatment was to release stress and thereby permit reshaping of the lenses.
The presence of the concentration gradients was found to be independent of whether the optical device had been heated above the strain point of the glass to relax induced stresses and alter the physical profile of the lenses (and hence increase their strengths). Inasmuch as the time necessary for stress relaxation is short compared with typical ion exchange reaction times, little effect on the concentration gradients results from the exposure to the high temperature. Consequently, it was possible to alternate various ion exchange treatments, above and below the strain point of the glass, with heat treatments above the strain point. Such modifications in processing variables permit the formation of lenses having desirable combinations of physical lens profile and concentration gradient.
The basic discovery of Borrelli et al. was that the ion exchange reaction can exert a substantive effect in reducing the radius of curvature of the lenses. That reduction, in turn, increases the optical power or strength of the lenses. No change in optical strength takes place unless the glass is heated above the glass strain point. Thus, the glass relaxes to an essentially stress-free state while the change in lens curvature occurs.
Borrelli et al. observed that the ion exchange reaction was effective to produce the beneficial changes in the lenses because the reaction proceeded more rapidly in the glass lenses than in the surrounding crystalline regions. Thus, little, if any, ion exchange takes place in the crystals.
Because where the ion exchange reaction is carried out at temperatures below the strain point of the glass the glass must thereafter be heated above its strain point, the preferred practice of Borrelli et al. was to conduct the ion exchange reaction at a temperature slightly above the strain point of the glass. Thus, the patentees employed temperatures of about 10.degree.-35.degree. C. above the strain point of the glass, i.e., temperatures up to about the annealing point thereof.
Inasmuch as U.S. Pat. No. 4,518,222 describes in considerable detail the process of, and the mechanisms involved in, ion exchange reactions in the development of lens arrays, the entire disclosure of that patent is incorporated herein by reference.
Microlens arrays have been produced commercially utilizing the disclosures of Bellman et al. and Borrelli et al. A significant limitation acting to curtail wider application of such arrays has been the inability to produce relatively short focal lengths in lenses having diameters of about 1-2 mm; i.e., lenses having focal lengths useful for applications requiring high numerical apertures. For example, in lenses having a diameter of 1 mm and larger, it has not been possible to decrease the radius of curvature sufficiently to produce axial heights therein of about 20 microns.
Accordingly, the primary objective of the present invention was to devise a method for forming optical devices composed of a photonucleable, crystallizable, lithium silicate glass body having at least one clear glass lens integral with and rising above at least one surface, said lens being surrounded by a crystallized glass matrix and having an axial height above the glass surface in excess of 100% greater than that present in the lenses produced by Bellman et al.
Another objective of the present invention was to devise means for producing lens arrays from photonucleable, crystallizable, lithium silicate glass bodies which had been exposed to short wave radiations in predetermined areas, heat treated to develop crystals in the exposed areas, and then ground and polished flat on one or both surfaces.